Question

Which solution is isotonic with $6$% W/V aqueous solution of urea?

[Mole mass of Urea$=60gm{mol}^{-1}$]

• A. $0.25M$ $NaCl$
• B. $0.5M$ $NaCl$
• C. $0.1M$ $NaCl$
• D. $1M$ $NaCl$

Answer 1

The correct option is B $0.5M$ $NaCl$

Two solutions are isotonic if they have the same osmotic pressure.

Osmotic pressure $\pi =iCRT$

For NaCl, the van 't Hoff factor $i=2$ as one NaCl molecule dissociates into 2 ions.

For urea, the van 't Hoff factor $i=1$ as urea does not dissociate.

6% W/V aqueous solution of urea means 6 g urea in 100 mL solution or

$ 60 g urea in 1000 mL (1 L) solution.

$C=\frac{\text{60 g}}{\text{60g/mol}\times 1L}=1M$

$i_{\text{urea}}\times C_{\text{urea}}=i_{\text{NaCl}}\times C_{\text{NaCl}}$

$1\times \text{1 M}=\text{2}\times C_{\text{NaCl}}$

$C_{\text{NaCl}}=0.5M$

Hence, option B is correct.